1. Field of the invention
The present invention relates to a semiconductor device including a semiconductor circuit constituted of a thin-film transistor (hereinafter, abbreviated as a TFT) and a manufacturing method thereof. More particularly, the present invention relates to an electro-optical device as represented by a liquid crystal display panel and an electronic device including such an electro-optical device as a constituent.
Throughout the specification, the term “semiconductor device” indicates all devices that utilize the semiconductor characteristics to function; electro-optical devices (hereinafter, referred to as display devices), semiconductor circuits and electronic devices are all included in the category of the semiconductor devices.
2. Description of the Related Art
Recently, there has been developed a technique for manufacturing a TFT by using a thin semiconductor film (with a thickness of about hundreds to thousands of nm) formed on a substrate which has an insulating surface. The TFT is widely applied to semiconductor devices such as an integrated circuit (IC) or an electro-optical device, and is urgently expected to be developed as, in particular, a switching element for a display device or the like.
An active-matrix liquid crystal display device is frequently used as a semiconductor device because images with high definition can be obtained as compared with a passive liquid crystal display device. The active-matrix liquid crystal display device includes: a gate wiring; a source wiring; a TFT in a pixel portion, which is provided at the cross point of the gate wiring and the source wiring; and a pixel electrode connected to the TFT in the pixel portion.
An amorphous silicon film is used as an amorphous semiconductor film for a conventional TFT because the amorphous silicon film can be formed on a large substrate at a low temperature of 300° C. or less. An inverse-stagger type TFT having a channel formation region formed of an amorphous semiconductor film is widely used.
Conventionally, a TFT is formed on a substrate by using five or more photomasks through a photolithography technique in an active-matrix electric device. The reduction of the number of manufacturing steps is believed to be effective to improve the productivity and the yield.
For the reduction of the number of manufacturing steps, it is necessary to reduce the number of photomasks used in the manufacture of the TFT. With the use of one photomask, the steps of resist application, prebaking, exposure, development, postbaking and the like, the preceding and following steps of forming a coating film, etching and the like, and, furthermore, the step of resist removal, washing and drying, are inevitably added to complicate the manufacture of the TFT.